I. Field of the Invention:
This invention relates generally to medical apparatus for monitoring blood flow in a blood vessel, and more particularly to an implantable Doppler flow meter for measuring the blood flow through an implanted vascular graft so that its patency can be periodically measured.
II. Discussion of the Prior Art:
In treating coronary artery disease, it is sometimes necessary to bypass one, two, three or four coronary arteries with an autologous graft, usually harvested from the patient's saphenous vein in the thigh or leg. The bypass graft is sutured or otherwise joined to the heart to act as a shunt around a blocked or partially blocked coronary artery.
It is important that the condition of the graft be monitored, post-surgery, to detect the further build-up of stenotic lesions or other obstructions to the flow of blood through that implanted graft. Various catheterization procedures are known for assessing the flow characteristics of a blood vessel or blood vessel graft. For example, using a thermal dilution technique, a catheter having two or more spaced-apart temperature sensitive elements thereon is routed through the vascular system to the site where the measurements are to be taken. Subsequently, a bolus injection of cold saline solution is introduced into the blood stream proximally of the measuring elements and then a measurement is taken of the time required for that cold liquid, driven by the blood, to pass between the spaced-apart temperature sensors. Knowing this time interval and other characteristics of the catheter, flow rates through the site can be measured and the condition of the vessel or graft inferred.
Another catheterization procedure involves the use of so-called impedance plethysmography. Here, a catheter having spaced-apart drive electrodes on either side of a pair of sense electrodes near the distal end portion thereof is routed through the vascular system to the site to be observed. A relatively high frequency alternating current signal is applied across two spaced-apart outer drive electrodes and the resulting signal developed across the pair of intermediate sensing electrodes is detected. The pulsating flow of blood through the site in question causes the sensed signal to be modulated and this modulating envelope signal corresponds to impedance swings which can be shown to be directly related to the volume of the blood vessel or graft being monitored.
It is also known in the art that piezoelectric transducers can be disposed on the distal end of a catheter and routed to a site of interest. One of the transducers is then driven by a signal generator located at the proximal end of the catheter to create an acoustic wave. Echo signals reflected back from blood cells to the pickup transducer creates an electrical signal which is fed to the receiver. Knowing the frequency of the acoustic energy and certain other parameters, the Doppler frequency shift can be detected to provide an indication of the rate of flow of blood through the site in question.
Further information concerning Doppler flow catheters can be gained from reading the following U.S. patents: Johnston U.S. Pat. Nos. 4,674,336; Johnston 4,637,401; Millar 4,665,925; and Abrams et al 4,671,295. The foregoing patents also contain references to still other patents and publications wherein Doppler catheters are used to measure blood velocity and flow rate in diseased arteries.
The introduction of catheters into the vascular system is not an altogether risk-free procedure. There are instances where the introduction of an angiographic catheter has damaged the endothelial lining of the blood vessel creating a flap which can block the vessel creating ischemia. Should this happen, it is again necessary to surgically repair the blood vessel and, in the case of coronary bypass surgery, would again require the patient's chest to be opened. Even if the blood vessels themselves are not damaged by the introduction of the catheter, there is always a chance that a particle of plaque may be dislodged from the interior walls of the blood vessel and may float to a point where a stroke or other life-threatening episode can occur due to a blockage in a small blood vessel.
Because of the above-described types of concerns, it has been difficult to obtain patient compliance with the need to periodically monitor the vascular graft over a period of time following its original implant.
It is accordingly a principal object of the present invention to provide an improved method and apparatus for monitoring the flow characteristics of an implanted blood vessel graft.
Another object of the invention is to provide an implantable system for monitoring blood flow through surgically implanted grafts which is substantially benigned.
Yet another object of the invention is to provide an implantable Doppler flow meter system which can readily be accessed on a routine basis to determine the state of the implanted grafts.